The present invention relates to the reduction of oxides of nitrogen, hereinafter referred to as NO.sub.x, in stack gases, particularly to gases resulting from the combustion of coal. The invention can also be used to reduce NO.sub.x from other combustion processes involving oil or natural gas. The invention is further related to a reduction of NO.sub.x using selective catalytic reduction in which ammonia, oxygen and a suitable catalyst are employed to reduce the NO.sub.x to water and nitrogen, and where the amount of ammonia escaping the process must be held to low values.
There are two approaches to controlling NO.sub.x emissions resulting from the combustion of hydrocarbon fuels. One is to control the combustion to limit the NO.sub.x formation and the second is to process the flue gas to reduce the NO.sub.x to nitrogen and water. One of the flue gas treating processes involves adding ammonia (NH.sub.3) as a reducing agent either with or without a catalyst for the selective reduction of the NO.sub.x to nitrogen and water. When a catalyst is used the process is referred to as selective catalytic reduction (SCR) and without a catalyst the process is referred to as selective noncatalytic reduction (SNCR). In the SNCR process there is a very narrow temperature window where NO.sub.x reduction occurs. This temperature is around 1740.degree. F. and if temperatures are reduced below about 1600.degree. F. the rate of reduction of NO.sub.x falls off drastically and the NH.sub.3 flows through the process without being reacted with the NO.sub.x. At temperatures higher than about 1900.degree. F. the injected NH.sub.3 burns to form additional NO.sub.x. The characteristics of the process are a high NH.sub.3 requirement and high NH.sub.3 concentrations in the treated gas. In order to reduce the optimum reaction temperature and broaden the temperature window, the use of catalyst has been incorporated with reduction processes using NH.sub.3. This combination reduces the temperature to a range of 550.degree. F. to 750.degree. F., which is convenient because it is the temperature of the gas leaving the economizer in a typical furnace. Various systems and processes are described in the prior art for carrying out a selective catalytic reduction of NO.sub.x, as well as various catalysts that can be used.
All of the above gas treating systems circulate or pass all of the stack gas through the catalyst bed and then discharge it to the atmosphere. No commercial SCR processes are used on coal-fired boilers in the U.S., although many large installations are in use in Japan. In Japan, the NO.sub.x is not normally reduced by more than 40 to 80 percent as it passes through the catalyst bed. Ammonia is added to the NO.sub.x in the flue gas in a mole ratio (NH.sub.3 /NO.sub.x) of approximately 0.45 to 0.85 to obtain the required reduction. After passing through the catalyst bed, the stack gas is normally passed to an air preheater or similar device to increase the overall efficiency of the boiler. On coal-fired systems this requires that the NH.sub.3 be held to a low level, typically 5 ppm or less, to prevent sticky fly ash deposits of ammonia salts and plugging in the air preheater. The low level of NH.sub.3 requires a large amount of catalyst to ensure complete reaction of NH.sub.3 and NO.sub.x .
From the above brief description of selective catalytic reduction systems on coal-fired boilers, it can be seen that the need to maintain the residual NH.sub.3 at low levels requires large amounts of catalyst. This is expensive both in capital cost and in operating cost, and it would be desirable to reduce these costs by reducing the amount of catalyst required.